Active matrix type image display apparatuses such LCD (liquid crystal display apparatus) and an organic EL (Electro Luminescence) display apparatus have been widely used because of their display performance, energy saving, etc. Such active matrix type display apparatuses have been mainstream display apparatuses for mobile phones, PDAs (Personal Digital Assistant), personal computers, laptop personal computers, television sets, and the like. In these display apparatuses, TFT substrates are generally used.
For instance, a liquid crystal display apparatus in which a display material such as liquid crystal is filled up between a TFT substrate and an opposite substrate, and which is constituted such that power voltage is selectively applied to the display material corresponding to each pixel. Here, the TFT substrate is a substrate on which a TFT (field-effect type thin film transistor) having an active layer composed of a semiconductor thin film (alternatively called as a semiconductor film) such as an amorphous silicon thin film or a polycrystalline silicon thin film is disposed. The above-mentioned image display apparatus is driven by the active matrix circuit of TFT. TFT substrates are also called as a “TFT arrayed substrate” because the TFT substrate has generally TFTs arranged in an array shape.
In a TFT substrate used for a liquid crystal display apparatus, etc., pairs of a TFT and one pixel in the screen of a liquid crystal display apparatus, which is called one unit, are arranged on a glass substrate in lengthwise and crosswise directions. In a TFT substrate, gate wirings are arranged on a glass substrate lengthwise with an equal space and source wirings or drain wirings are arranged crosswise with an equal space, for example. Further, a gate electrode, a source electrode and a drain electrode are provided in the above-mentioned unit which constitutes each pixel, respectively.
Here, the production of a transistor using the above-mentioned silicon thin film has problems in safety and costs of equipment since the production uses a silane-based gas. Further, a TFT formed from an amorphous silicon thin film has an electron mobility of as low as about 0.5 cm2/Vs and a small band gap. As a result, it may malfunction when absorbing visible light. Furthermore, production of a polycrystalline silicon thin film requires a heat treatment at a relatively high temperature so that an energy cost is expensive, and it is difficult to form the polycrystalline silicon thin film directly on a large-size glass substrate. Further, it is difficult to pattern silicon-based semiconductors by wet etching. As a result, dry etching which requires a high cost of equipment and has low productivity has to be employed.
In this connection, developments of TFTs using an oxide semiconductor thin film, which can be formed at a lower temperature and has a high electron mobility, are actively proceeded with (see Patent Documents 1 to 6, for example).
In general, the larger the overlap of the s orbit of a metal ion becomes, the higher the electron mobility of an oxide crystal. Oxide crystals of Zn, in and Sn having a larger atomic number have a high electron mobility of 0.1 to 200 cm2/Vs. Further, in an oxide, oxygen and a metal ion are bonded through an ionic bond, and the chemical bond has no orientation. Therefore, even if the oxide is in an amorphous state wherein orientation of the bonds is not uniform, the oxide can have an electron mobility near that of an oxide in a crystalline state.
From this fact, a transistor having a high field-effect mobility can be produced using a metal oxide even which is amorphous, as is different from a Si-based semiconductor. Namely, by utilizing the above-mentioned properties, semiconductor devices, circuits or the like using a crystalline or amorphous metal oxide containing Zn, In and Sn are being studied.
In the technique using metal oxides, an oxide thin film containing in, Ga and Zn, or an oxide thin films containing Zn, In and Sn are being studied about their conductivity or etching properties with oxalic acid (see Non-patent Document 1 and Patent Documents 7 to 9, for example).
Also, in the technique concerning a transparent conductor film, selective etching in the case where lanthanoid elements are added is being studied (Patent Document 10).
[Patent Document 1] JP-A-2006-165527
[Patent Document 2] JP-A-H11-505377
[Patent Document 3] JP-A-560-198861
[Patent Document 4] JP-A-2006-528843
[Patent Document 5] JP-A-2006-502597
[Patent Document 6] WO2005/088726
[Patent Document 7] JP-A-H10-83719
[Patent Document 8] JP-A-2000-256061
[Patent Document 9] JP-A-2006-210033
[Patent Document 10] JP-A-2004-240091
[Non-patent Document 1] Philosophical Magazine B,01 May, 2001 (01.05.01), Vol. 81, No. 5, pages 501-515. ORITA et al.